Triggers & Active Data Bases

Triggers What is a trigger? Trigger is like a procedure that is automatically invoked by the DBMS in response to specified changes to data base Trigger is like a ‘Daemon that monitors a data base, and is executed when the data base is modified in a way that matches the event specification A data base that has a set of associated triggers is called an active data base

Trigger Parts Event A change to data base that activates the trigger Restriction A trigger restriction specifies a Boolean (logical) expression that must be TRUE for the trigger to fire Action A procedure that is executed when the trigger is activated. Similar to stored procedures, a trigger action can contain PL/SQL statements

Types of Triggers Row Triggers A row trigger is fired each time the table is affected by the triggering statement. If a triggering statement affects no rows, a row trigger is not executed at all. Statement Triggers A statement trigger is fired once on behalf of the triggering statement, regardless of the number of rows in the table that the triggering statement affects (even if no rows are affected)

Trigger Timings Before Trigger Execute the trigger action before the triggering statement. Eliminate unnecessary processing of the triggering statement. After Trigger AFTER triggers are used when you want the triggering statement to complete before executing the trigger action

Trigger CREATE or REPLACE TRIGGER cs348 after INSERT ON weatherforecast FOR EACH ROW WHEN (:new.temp>= 60) BEGIN DBMS_OUTPUT.PUT_LINE(‘NICE WEATHER’); END cs348 / Show error;

Triggers can be confusing Update table weatherforcast T1 Fired Before Update on weatherforcast For each row Begin Insert into weatherforcast2 values (..,..); END; T2 Fired Before Update on weatherforcast 2 For each row Begin Insert into weatherforcast3 values (..,..); END;

Triggers ItemIdquantitycustomeridunitprice ItemIdunitprice 123$440 What are the uses of triggers? Flexible Management of integrity Trigger Fired ItemIdquantitycustomeridunitprice Log generation to support auditing & security Prevent invalid transactions

Thank You

References [1] [2] Database Management Systems, (NOTE: Third Edition), by Raghu Ramakrishnan and J. Gehrke. [3]